The present invention relates to the process of the production of polyhydric alcohols from their monomers. Examples of monomers that could be subjected to this process are glycerol, ethylene glycol, propylene glycol, sorbitol, sucrose, D-glucose and fructose. This list is by no means exhaustive. Polyglycerol is defined as a polymer containing two or more units of glycerol and it may be either linear or branched.
Numerous methods are known for the preparation of polyglycerols with earlier works evolved around the use of thermal dehydration of glycerol. The polymerisation was carried out at atmospheric pressure and at an elevated temperature, which is about 270° C. −280° C. as mentioned in U.S. Pat. No. 2,487,208. The process can be accomplished without the use of catalyst but the yield of polyglycerol is considerably low as reported by Hauschild and Petit (1956). Therefore, various catalysts have been introduced to aid in the formation of polyglycerol such as mixtures of sulphuric acid and triacetin as described in U.S. Pat. No. 3,968,169, hypophosphorus acid with sodium hydroxide as appeared in U.S. Pat. No. 4,551,561, alkaline carbonates such as potassium carbonate with aluminium oxide as in J.P. No. 61,238,749 and sodium or potassium hydroxide as in U.S. Pat. No. 5,710,350.
Polyglycerol formation was also reported with either solketal, glycidol or glycerol carbonate as the reactants when reacted with hydrotalcite at elevated temperatures as described in W.O. Pat. No. 9,516,723. Other than that, rubidium, caesium and potassium fluoride salts on alumina or zeolites were used as catalyst for the polymerisation of glycerol. In addition, glycidol, glycerol carbonate and solketal were polymerised using the above fluoride salts into polyglycerol as reported in W.O. Pat. No. 9,521,210. Other than that, in U.S. Pat. No. 5,635,588, both linear and cyclic polyglycerols were products of reaction between glycidol, glycerol carbonate and solketal with beta-zeolites as catalysts.
While some other literatures reported the use of epichlorohydrin in the process to prepare polyglycerol, in U.S. Pat. No. 4,960,953, Jakobson et. al. disclosed a process to produce polyglycerol, which comprised reacting glycerol, diglycerol or higher polyglycerol with epichlorohydrin at 90° C. to 170° C. to produce a crude chlorohydrin/ether mixture, followed by adding an amount of strong base at least substantially equivalent to the organically bound chlorine content of the chiorohydrin/ether mixture, and desalting the mixture and recovering the glycerol, diglycerol and higher polyglycerol fractions.
Allyl alcohol is another route in preparing polyglycerols. The process involve depoxidation of the ally/alcohol, in which glycidol would be formed and then followed by polymerisation of the glycidol. This was proven as another effective method to prepare polyglycerol as shown in J.P. No. 2,169,535.
Despite the fact that the background art in preparing polyglycerol is crowded and diverse, it is evident that the synthesis of polyglycerol and diglycerol from glycerol has one major drawback, which is the duration of reaction. It is a usual practice to have a reaction time of minimum 5 hours to 72 hours to carry out polymerisation with a mixed yield of glycerol, digycerol, triglycerol and other higher polyglycerols. Other preparation such as those that involve epichlorohydrin may polymerise at a faster rate, but polyglycerols produced from epichlorohydrin are not particularly favoured by the industry, as there may still be organically bound chlorine in the polyglycerols.